Sixty-four Holstein steers with an average weight of 247 ± 4 kg were used in a 70-d experiment to evaluate the effects of partial replacement of steam flaked corn (SFC) with extruded corn meal (EC; Matrix Nutrition, Chandler, AZ) in a finishing diet on growth performance and dietary net energy. Steers were blocked by initial weight into four weight groupings, and randomly assigned within weight groupings to 16 pens (4 steers per pen).Pens were 43 m² with 22 m² of overhead shade, automatic waters, and 2.4 m fence-line feed bunks. Treatments consisted of a steam flaked corn-based finishing diet containing 0%, 10%, 20%, or 30% EC, where EC replaced 0%, 15%, 30%, or 45% of SFC (DM basis). The SFC was prepared as follows. A chest situated directly above the rollers (46 × 61 cm corrugated) was filled with corn and then brought to a constant temperature at atmospheric pressure of 102˚C using steam. Corn was steamed for approximately 20 min before starting the rollers. The first approximately 441 kg of steam-flaked grain was allowed to pass from the rollers before material was collected for use in the trial. This preliminary period served for warming the rolls and for adjusting the tension on the rolls to provide a flake density of 0.31 kg/L (24 lbs/bushel). The SFC was then allowed to air-dry for 3 days before feeding. Ingredient and nutrient composition of experimental diets are shown in Table 1. Diets were prepared at weekly intervals and stored in plywood boxes located in front of each pen. Steers were allowed ad libitum access to their experimental diets. Fresh feed twice daily at 06:00 and 14:00 h, allowing for approximately 5% residual. All steers were provided ad libitum access to water. Individual steers were weighed upon initiation and completion of the trial. In the calculation of steer performance live weights were reduce 4% to adjust for digestive tract fill. Estimates of steer performance were based on pen means.

Energy gain (EG) was calculated by the equation: EG = ADG^1.097×0.0557 W^0.75, where EG is the daily energy deposited (Mcal/d), W is the mean shrunk body weight (kg; [11] ). Maintenance energy (EM) was calculated by the equation: EM = 0.084 W^0.75 [12] . Dry matter intake (DMI) is related to EM and dietary NEm according to the equation: DMI = EM/NEm + EG/(0.877 NEm − 0.41), and can be resolved for estimation of dietary NEm by means of the quadratic formula: x = [−b ± (b² − 4ac)^0.5]/2a , where x = NEm, a = −0.877 DMI, b = 0.877 EM + 0.41 DMI + EG, and c = −0.42 EM [13] . Dietary net energy for gain (NEg) was derived from NEm by the equation: NEg = 0.877 NEm − 0.41 [13] .

Pens were used as experimental units. The experimental data were analyzed as a randomized complete block design experiment according to the following statistical model:

Y_ij = μ + B_i + T_j + E_ij

where μ is the common experimental effect, B_i represents initial weight group effect (df = 3), T_j represents dietary treatment effect (df = 3), and E_ij represents the residual error (df = 9). Treatments effects were tested using the linear, quadratic and cubic polynomials (Statistix 10, Analytical Software, Tallahassee, FL).

Four Holstein steers (210 ± 7 kg) with cannulas in the rumen (3.8 cm internal

¹DMB = Dry matter basis; ²Analyzed chemical composition for crude protein, NDF, starch and soluble (amyloglucosidase reactivity) starch were 8.4%, 10.9% and 72.4%, and 8.6% (DM basis), respectively; ³Analyzed chemical composition for crude protein, NDF, starch and soluble (amyloglucosidase reactivity) starch were 8.7%, 10.3%, 69.8% and 30.9% (DM basis), respectively; ⁴Trace mineral salt contained: CoSO₄, 0.068%; CuSO₄,1.04%; FeSO₄, 3.57%; ZnO, 1.24%; MnSO₄, 1.07%; KI, 0.052%; and NaCl, 92.96%; ⁵Based on tabular values for individual feed ingredients [12] .

diameter) and proximal duodenum [14] were used in a 4 × 4 Latin square experiment to evaluate treatment effects on characteristics of ruminal and total tract digestion. Dietary treatments were the same as indicated for Trial 1 (Table 1) with the inclusion of chromic oxide (2.5 g/kg) as a digesta marker. Steers were maintained in individual pens (5.6 m²) with automatic waters. Diets were fed at 0800 and 2000 h daily. In order to avoid the complications of feed refusals, DMI was restricted to 4.5 kg/d. Experimental periods were 14 d, with 10 d for dietary treatment adjustment and 4 d for collection. Between each experimental period there was a 7-d time-frame to recovery during which all steers were fed the control diet (0% extruded corn meal diet). During collection, duodenal and fecal samples were taken twice daily as follows: day 1, 0750 and 1350 h; day 2, 0900 and 1500 h; day 3, 1050 and 1650 h, and day 4, 1200 and 1800 h. Individual samples consisted of approximately 700 mL of duodenal chyme and 200 g (wet basis) of fecal material. Samples from each steer within each collection period were composited for analysis. During the final day of each collection period, ruminal samples were obtained from each steer via ruminal cannula 4 h after feeding. Ruminal fluid pH was determined on fresh samples. Samples were strained through 4 layers of cheesecloth. Two milliliters of freshly prepared (25 g/100 mL) meta-phosphoric acid was added to 8 mL of strained ruminal fluid. Samples were then centrifuged (17,000 × g for 10 min), and supernatant fluid was stored at −20˚C for VFA analysis. Upon completion of the experiment, ruminal fluid was obtained via the ruminal cannula from all steers and composited for isolation of ruminal bacteria by differential centrifugation [15] .

Corn, feed and fecal samples were subjected all or part of the following analysis: DM (oven drying at 105˚C until no further weight loss); ash (method 942.05, AOAC [16] ), Kjeldahl N (method 984.13, AOAC [17] ); aNDFom (Van Soest et al. [18] corrected for NDF-ash, incorporating heat stable α-amylase (Ankom FAA, Ankom Technology, Macedon, NY) at 1 mL per 100 mL of NDF solution); chromic oxide [19]; starch content and changes in starch solubility as a result of corn processing were determined using amyloglucosidase [20] . Duodenal samples were subjected the following analysis: DM (oven drying at 105˚C until no further weight loss); ash (method 942.05, AOAC [16] ), Kjeldahl N (method 984.13, AOAC [17] ), ammonia N (method 941.04, AOAC [17] ); aNDFom (Van Soest et al. [18], corrected for NDF-ash, incorporating heat stable α-amylase (Ankom FAA, Ankom Technology, Macedon, NY) at 1 mL per 100 mL of NDF solution); purines [21]; chromic oxide [19]; and starch [20] . Concentrations of VFA in ruminal fluid were determined by gas chromatography using a DB-FFAP Megabore column (30 m × 0.530 mm, J & W Scientific, Folsom, CA). Column, inlet and detector temperatures were maintained at 130˚, 195˚, and 200˚C, respectively, with carrier gas (N) flow rate at 20 mL/min.

Total DM flow to the duodenum and fecal excretion were estimated based on marker ratio: the relationship of Cr intake (g) vs. concentration of Cr in duodenal and fecal samples. Microbial organic matter (MOM) and N (MN) leaving the abomasum was calculated using purines as a microbial marker [21] . Organic matter fermented in the rumen was considered equal to OM intake minus the difference between the amount of total OM reaching the duodenum and MOM reaching the duodenum. Feed N escape to the small intestine was considered equal to total N entering the duodenum minus ammonia-N, MN and endogenous N (0.195 × BW^0.75, Ørskov et al. [22] ). Methane production (mol/mol glucose equivalent fermented) was estimated based on the theoretical fermentation balance for observed molar distribution of VFA [23] .

The effects of extruded corn meal level in diet (0, 10, 20, or 30%) on characteristics of digestion in cattle were analyzed as a balanced 4 × 4 Latin square design experiment:

Y_ijk = µ + S_i + P_j + T_k + E_ijk,

where: Y_ijk is the response variable, µ is the common experimental effect, S_i is the steer effect, P_j is the period effect, T_k is the treatment effect and E_ijk is the residual error. Treatments effects were tested by means of linear and quadratic polynomials (Statistix 10, Analytical Software, Tallahassee, FL).